The present invention relates to a copier, facsimile apparatus, printer or similar image forming equipment and, more particularly, to a device incorporated in such equipment for adjusting the output of an image density sensor which optically senses the density of a toner image formed on a photoconductive element and representative of a reference pattern.
A prerequisite with image forming equipment of the type forming a toner image on a photoconductive element is that the image density and contrast be controlled to desirable levels in order to insure high image quality. The image density and contrast are effected by the toner concentration of a developer, bias voltage for development, lamp voltage for exposure, etc. To meet the above requirement, one conventional image forming apparatus forms a toner image representative of a reference pattern on a photoconductive element, determines the density of the toner image by use of an image density sensor, and controls, for example, the toner concentration or the bias voltage, as needed. However, the problem is that the output of the sensor is susceptible to the surface configuration and eccentricity of the photoconductive element (distance to the sensor), toner particles depositing on and smearing the light-sensitive surface of the sensor, a voltage saturation range particular to the conversion of the output current of the light emitting element (e.g. phototransistor) to a voltage, temperature drift, etc. It is likely, therefore, that the output of the sensor is deviated from a desirable range of output characteristic, resulting in erroneous control. In light of this, it has been customary to sense the background of the photoconductive element together with the toner image of interest, compare the difference between the resulting two outputs and the difference between their reference outputs, control the toner concentration of a developer on the basis of the result of comparison, and thereby adjust the output of the sensor against the smears caused on the light-sensitive surface of the sensor by the toner. This kind of implementation is disclosed in, for example, Japanese Patent Laid-Open Publication No. 53869/1984.
However, the above-stated scheme determines whether or not to adjust the sensor output by using a single threshold, i.e., the difference between reference outputs. This brings about a problem that even an output little different from the threshold is immediately corrected, and the correction is effected due to the influence of the eccentricity of the photoconductive element or noise. To eliminate this problem, Japanese Patent Application No. 134674/1989 teaches a procedure consisting of repetitively detecting the background of the photoconductive element a predetermined number of times while determining whether or not the resulting output lies in a predetermined range each time, and adjusting the sensor output when it is not found in the predetermined range. Since this procedure repeats the detection a predetermined number of times without exception, even a sensor output which is far different from the target output and almost saturated is apt to continuously hold until the detection has been repeated the fixed number of times, obstructing accurate control over the toner concentration and other subjects. In addition, assume equipment of the kind displaying an error when the adjusted output does not fall in the predetermined range, and a sensor extremely susceptible to, for example, temperature drift. Then, since the sensor output noticeably varies with the temperature inside the equipment, the equipment is apt to display an error frequently.